This invention relates to synthetic cork closures for liquid containers and to processes for making such closures.
Synthetic corks are typically made from a foamed polymer and are formed using either a profile extrusion method or injection molding or may be punched out of foamed sheets. Hybrid corks also exist where natural cork is ground and recombined using binding agents. Synthetic corks and processes for preparing them are described in U.S. Pat. Nos. 5,975,322, 5,904,965, 5,855,287, 5,710,184, 5,496,862 and 4,363,849. All of these synthetic corks and hybrid corks suffer from the uncontrolled permeation of gases in and out of the cork and the scalping of flavors caused by the polymers used.
It would be desirable to provide synthetic corks that have high resistance to the permeation of gases in and out of the cork.
In a first aspect, the present invention is a synthetic cork closure having at least a portion thereof coated with a gas impermeable polymer coating composition.
In a second aspect, the present invention is a process for preparing the closure of the first aspect which comprises providing a synthetic cork closure and coating at least a portion thereof with a gas impermeable polymer coating composition.
In a third aspect, the present invention is a process for preparing the closure of the first aspect which comprises inserting the synthetic closure into a container, dripping the gas impermeable coating composition onto the free end of the closure and allowing the solvent in the coating composition to evaporate.
The synthetic cork closures which can be employed in the practice of the present invention include the synthetic closures described in U.S. Pat. Nos. 5,975,322, 5,904,965, 5,855,287, 5,710,184, 5,496,862 and 4,363,849, incorporated herein by reference. Most preferred synthetic cork closures are those which are commercially available from Neocork Technologies, Inc., Napa Valley, Calif.
The gas impermeable coating composition of the present invention can be prepared by dissolving a gas impermeable polymer in a volatile organic solvent solution. Preferably, the organic solvent solution comprises a blend of a solvent and a cosolvent. The organic solvent dissolves the gas impermeable polymer. The cosolvent is employed to control solvent evaporation rate and to dissolve any additives which can not be dissolved by the solvent. If the gas impermeable polymer is vinylidene chloride polymer, the cosolvent is also used to control the vinylidene chloride polymer crystallinity.
The gas impermeable polymers which can be employed in the practice of the present invention include vinylidene chloride polymers and copolymers, polychlorotrifluoroethylene, polyethylene terephthalate and polyisobutylene. The most preferred gas impermeable polymers are vinylidene chloride polymers.
The vinylidene chloride polymers which can be employed in the practice of the present invention are well-known in the art. See, for example, U.S. Pat. Nos. 3,642,743 and 3,879,359. The most common PVDC resins are known as SARAN(trademark) resins, manufactured by The Dow Chemical Company. As used herein, the term xe2x80x9cvinylidene chloride polymerxe2x80x9d or xe2x80x9cPVDCxe2x80x9d encompasses homopolymers of vinylidene chloride, and also copolymers and terpolymers thereof, wherein the major component is vinylidene chloride and the remainder is one or more monoethylenically unsaturated monomers copolymerizable with the vinylidene chloride monomer. Monoethylenically unsaturated monomers which can be employed in the practice of the present invention for preparing the vinylidene chloride polymers include vinyl chloride, alkyl acrylates, alkyl methacrylates, acrylic acid, methacrylic acid, itaconic acid, acrylonitrile, and methacrylonitrile. Preferred ethylenically unsaturated monomers include vinyl chloride, acrylonitrile, methacrylonitrile, alkyl acrylates, and alkyl methacrylates. More preferred ethylenically unsaturated monomers include vinyl chloride, acrylonitrile, methacrylonitrile, and the alkyl acrylates and alkyl methacrylates having from 1 to 8 carbon atoms per alkyl group. Most preferred ethylenically unsaturated monomers are vinyl chloride, methyl acrylate, ethyl acrylate, and methyl methacrylate, acrylonitrile, and methacrylonitrile.
Preferably, the vinylidene chloride copolymer is (1) a copolymer of (a) from about 80 to about 93 mole percent vinylidene chloride and (b) from about 20 to about 7 mole percent of at least one monoethylenically unsaturated monomer copolymerizable therewith (SARAN F-278) or (2) a copolymer of (a) from about 65 to about 75 mole percent vinylidene chloride and (b) from about 35 to about 25 mole percent of at least one monoethylenically unsaturated monomer copolymerizable therewith (SARAN F-310).
The above vinylidene chloride polymers and processes for preparing them are well known. See, for example, U.S. Pat. Nos. 3,817,780; 3,879,359; 4,351,929 and 4,451,632, incorporated herein by reference.
The organic solvents which can be employed in the practice of the present invention for preparing the solvent mixture include acetone, methyl ethyl ketone, cyclohexanone, ethyl acetate, n-propyl acetate, isopropyl acetate, propylene oxide, dioxane, tetrahydrofuran and mixtures thereof. Preferred solvents are methyl ethyl ketone, acetone, ethyl acetate and tetrahydrofuran. More preferred solvents are methyl ethyl ketone, acetone, and tetrahydrofuran, with methyl ethyl ketone as the most preferred. Cosolvents which can be employed in the practice of the present invention for preparing the solvent mixture include aliphatic and alicyclic hydrocarbons, such as hexane, heptane, cyclohexane, cyclohexene, methylcyclohexane and dialkylethers, such as t-amyl methyl ether. Most preferred cosolvents are methylcyclohexane and heptane. Most preferred is methylcyclohexane.
A commonly known organic solvent system, which is a mixture of the solvent THF (tetrahydrofuran) and the cosolvent toluene, can be employed in the practice of the present invention for preparing the gas impermeable coating composition.
The coating composition may also contain various additives to impart desirable properties such as, for example, slip properties, to the finished coating.
The amount of solvent and cosolvent used in the solvent mixture depends on many factors such as the composition and solubility of vinylidene chloride polymer, the desired flow characteristics of the coating, desired pot life and drying time, desired coating thickness, and desired wettability of the closure the type of cosolvent employed, vinylidene chloride crystallinity and the temperature at which the vinylidene chloride and other additives are dissolved. In general, the solvent is used in an amount of from about 50 to 85 weight percent, preferably from about 60 to about 80 weight percent and, most preferably, in an amount of from about 65 to about 75 weight percent, based on the weight of the solvent mixture and the balance is the cosolvent.
The thixotropic agent which can be employed in the practice of the present invention include fumed silica, kaopolite, bentonites, talc, or mixtures thereof.
The amount of thixotropic agent employed in the practice of the present invention for preparing the coating composition depends on desired flow characteristics of the coating, the specific method of applying the coating, and the desired coating thickness.
In general, this amount is from about 0 to about 25 wt. % based on the weight of the composition. Preferably, the gas impermeable polymer coating composition comprises from about 5 weight percent to about 20 weight percent of a vinylidene chloride polymer, from about 70 weight percent to about 90 weight percent of an organic solvent or blend of organic solvents and, from about 5 weight percent to about 10 weight percent of a thixotropic agent. The coating composition can be prepared by methods known in the art such as by dissolving a vinylidene chloride polymer or resin in a volatile organic solvent mixture at a temperature and for a time sufficient to dissolve substantially all of the vinylidene chloride resin.
The synthetic cork closure of the present invention can be prepared by providing a synthetic cork closure and applying a coating of the gas impermeable polymer composition to at least a portion of the surface thereof. For example, the coating may be applied only to the surface of the closure that is likely to contact the contents of the container, such as one or both ends of the synthetic cork closure, or it can be applied to the whole surface of the cork closure.
The coating composition can be applied onto the surface of the synthetic cork closure by methods known in the art such as, for example, by analox gravure coating, offset coating, pad print coating, screen or stencil coating, brush coating, and spray coating. In gravure coating, the coating composition is doctored onto an inert surface and either pressed onto the closure or transferred to an offset pad, which in turn is used to apply the coating to the closure. Likewise, a pad print system can be used to transfer the coating in a controlled amount from a cliche to the closure by means of an elastomeric pad. Brush and spray coating are very conventional techniques which can be used to apply the coating to the closure, albeit in a less-controlled manner. Another method is applying the coating by means of a silk-screen or stencil process involving a squeegee. Depending on the solvent system, for any of these methods, the coating can be applied from an open well or a closed cup system. These are all methods well-known to those skilled in the art of applying coatings to surfaces.
The coating composition can be applied onto the surface of the synthetic cork closure after it has been inserted into a container. For example, the coating composition, which comprises a vinylidene chloride copolymer dissolved in a solvent, can be applied to the inserted closure by dripping the coating composition onto the free end of the closure and allowing the solvent to evaporate.
Any residual solvent in the coating can be removed by conventional methods such as, for example, by drying the coated substrate in a forced-air oven. Advantageously, the coated substrate is dried for a period of time and at a temperature sufficient to remove the solvent and develop crystallinity, if required, in the finished coating. The temperature and length of time can be easily determined by those skilled in the art without undue experimentation. The following examples are for illustrative purposes only and are not intended to limit the scope of this invention. Unless otherwise indicated, all parts and percentages are by weight.